Basement membranes are thin sheets of extracellular matrix that surround all epithelia, endothelia, peripheral nerves, muscle cells, and fat cells. They are thought to play roles in filtration, in tissue integrity and compartmentalization, and in cell adhesion, proliferation, migration, and differentiation. Defects in basement membranes are responsible for a diverse array of human diseases, and the phenotypes of knockout mice show that basement membranes play critical roles during development and in mature tissues. The broad, long term goals of the research proposed here are to understand 1) the functions of basement membranes in mammalian development and physiology, and 2) how heterogeneity in basement membrane composition translates into functional specificity in vivo. The Specific Aims will employ conventional transgenic and knockout mice in novel approaches to study basement membranes in which it is found. We have described a null mutation in Lama5 which results in embryonic lethality. Here, the embryonic lethality will be bypassed, first by producing mosaic mice by aggregating Lama5 -/- embryos with wild-type embryos. The resulting chimeras will contain ~50 percent wild-type cells that will provide normal laminin alpha5 protein. Second, we will generate a conditional G domain-specific mutation in Lama5 which deletes the G domain. Aim 3 seeks functions for the laminin alpha1 chain through the analysis of Lama1 knockout mice. This chain has been shown to have multiple functions in vitro, but there is no evidence for specific functions in vivo. Both absolute and conditional knockouts will be studied, as will chimeric mice containing variable percentages of mutant cells. The results of these studies will lead to new information regarding the assembly and function of basement membranes and of their diverse components in vivo.